1. Field of the Invention
This invention relates to the use of cerium oxide (CeO.sub.2) for the removal of chlorine (Cl.sub.2) from reducing gases. The cerium chloride (CeCl.sub.3) generated by the reaction of the Cl.sub.2 with CeO.sub.2 can be regenerated back to CeO.sub.2 by exposure to an oxidizing atmosphere such as air with the release of Cl.sub.2.
2. Description of the Prior Art
For purposes of illustration the removal of chlorine from fuel gases generated in a gasifier from hydrocarbons which also contain either Cl.sub.2 or compounds which contain chlorine that dissociate with the release of Cl.sub.2 will be used. Many of the hydrocarbons that may be used for the creation of fuel gases also contain sulfur which is found in the fuel gas mainly in the form of hydrogen sulfide (H.sub.2 S) or carbon oxysulfide (COS). Patents currently exist for the removal of sulfur from fuel gases with cerium oxide and doped cerium oxide, and thermodynamic calculations indicate that the removal of H.sub.2 S, COS, and Cl.sub.2 can occur simultaneously. However, the use of fuel gases produced from hydrocarbons containing both sulfur and chlorine for illustrative purposes does not preclude the use of the technology for the reduction of Cl.sub.2 from other reducing gases containing Cl.sub.2 no matter what their method of preparation or production.
One of the major applications of the technology may be the removal of Cl.sub.2 from the fuel gases used in integrated-gasifier-combined cycle (IGCC) power plants which may become the preferred method of electric power generation in the future. The gasifiers of IGCC systems may operate at temperatures as high as 2800.degree. F. Undesirable components of the fuel gases produced in the gasifier may be removed by lowering the temperature of the fuel gases to a temperature either slightly above or slightly below the boiling point of water by methods known to those skilled in the art and then exposing the fuel gases to chemicals and water solutions of chemicals capable of removing the undesirable components such as Cl.sub.2 and H.sub.2 S from the gases. Because of the almost complete loss of the sensible heat from the gases, the cost of electricity (COE) of power plants which utilize this technology for removal of undesirable components from the fuel gases produced by gasifiers has been computed to be about 10% higher than the cost of IGCC systems that could remove the undesirable components such as H.sub.2 S and Cl.sub.2 at high temperatures. Although methods have been developed which operate at close to the operating temperature of the gasifier for removing H.sub.2 S from fuel gases by exposing them to materials such as zinc ferrite, zinc titanate, copper-manganese oxide mixtures, and cerium oxide, an extensive literature review reveals no studies of methods for the use 9f solid sorbents to remove Cl.sub.2 from fuel gases.
Data available on the analysis of coals produced in Illinois show that the chlorine content of those coals can range from 0.02% to 0.42% with the average being 0.112%. The sulfur content of these same coals ranges from 1.14% to 4.52% with the average being 3.31%. When such coals are used in an oxygen blown gasifier the H.sub.2 S content of the resulting gases based on their average sulfur content would be about 1.00%. If all of the chlorine in the coal is converted to Cl.sub.2, the Cl.sub.2 content of the fuel gases produced from the coal whose chlorine content is 0.42% could be as high as 0.10% or 1000 ppm.
In an IGCC system the fuel gases from the gasifier are fired into a gas turbine, and it is recognized that the blades in a gas turbine may operate at temperatures as high as 2000.degree. F. (1093.3.degree. C.). When the temperature of the blades in a gas turbine approaches 2000.degree. F. in the oxidizing atmosphere created by burning the fuel gases, the blades may be partially oxidized. As a result of this oxidation there is a film of oxide coating the blades. The oxides on turbine blades are in many cases either high in Al.sub.2 O.sub.3 or Cr.sub.2 O.sub.3 because many turbine blade alloys contain aluminum and chromium.
TABLE I ______________________________________ SUMMARY OF MELTING POINT AND VAPOR PRESSURE Fluorides Chlorides Oxides mp tv mp tv mp ______________________________________ FeX.sub.2 1020 906 676 536 FeO 1378 FeX.sub.3 1027 673 303 167 Fe.sub.2 O.sub.3 1594 NiX.sub.2 1450 939 1030 607 NiO 1955 CoX.sub.2 1250 962 740 587 CoO 1805 CrX.sub.2 894 928 820 741 CrX.sub.3 1404 855 1150 611 Cr.sub.2 O.sub.3 2400 AlX.sub.3 1273 825 193 67 Al.sub.2 O.sub.3 2050 ______________________________________ mp = melting point (.degree.C.) tv = temperature at which v.p = 10.sup.-4 atmospheres
The data in Table I shows that all of the chlorides of the major constituents of turbine blades have substantial vapor pressures at temperatures less than the operating temperatures expected in many gas turbines with AlCl.sub.3 having a very low melting point and a very high vapor pressure. As a result, there will be a continuous vaporization of the chlorides formed from the oxides created on the turbine blades when the fuel gases contain Cl.sub.2. This will result in premature failure of the turbine blades.
A need therefore exists for a method for removing chlorine from fuel gases produced from coals containing chlorine containing compounds or chlorine itself.